Write an Equation to Describe the Variation $\mathrm { s }$

Write an equation to describe the variation. Use k for the constant of proportionality.
- $\mathrm { s }$ varies directly as $\mathrm { m }$

A) $\mathrm { s } = \frac { \mathrm { k } } { \mathrm { m } }$
B) $\mathrm { m } = \frac { \mathrm { k } } { \mathrm { s } }$
C) $\mathrm { s } = \mathrm { km }$
D) $\mathrm { m } = \mathrm { ks }$